A fuel cell is a kind of electricity generating apparatus that extracts electric energy by electrochemically oxidizing a fuel such as hydrogen or methanol and, in recent years, is drawing attention as a clean energy supply source. In particular, a polymer electrolyte fuel cell, because of its standard operating temperature being as low as about 100° C. and its energy density being high, is expected to be widely applied as distributed electricity generating facilities on relatively small scales or electricity generating apparatuses for mobile units such as motor vehicles or ships and boats. Furthermore, the polymer electrolyte fuel cell is also drawing attention as electricity sources for small-size mobile appliances and portable appliances and is expected to be mounted in cellular phones, personal computers and the like, replacing the secondary batteries such as nickel hydride batteries and lithium-ion batteries.
A fuel cell is usually constituted of a cell provided as a unit in which electrodes, an anode and a cathode, on which reactions responsible for electricity generation and a polymer electrolyte membrane that becomes a proton conductor between the anode and the cathode constitute a membrane electrode assembly (hereinafter, sometimes referred to simply as MEA) and the MEA is sandwiched between separators. A main component of the polymer electrolyte membrane is an ionic group-containing polymer (polymer electrolyte material). To increase durability, a polymer electrolyte composition compounded with an additive and the like may also be used as the main component. The polymer electrolyte composition is also suitable as a binder or the like in an electrode catalyst layer for use in a particularly severely oxidizing atmosphere. As for required characteristics of the polymer electrolyte membrane and the polymer electrolyte composition, high proton conductivity is first cited. Particularly, having a high proton conductivity even in a high-temperature low-humidified condition is needed. Furthermore, the polymer electrolyte membrane and the polymer electrolyte composition are responsible for a function as a barrier that prevents a direct reaction between fuel and oxygen and therefore are required to have low permeability to the fuel. Furthermore, the polymer electrolyte membrane and the polymer electrolyte composition also need to have a chemical stability to withstand a strong oxidizing atmosphere during operation of the fuel cell and a mechanical strength and a physical durability that enable the withstanding of thin membrane formation and repetitions of swelling and dryness and the like.
So far, as the polymer electrolyte membrane, Nafion (registered trademark) (made by DuPont company), which is a perfluorosulfonic acid based polymer, has been widely used. Nafion (registered trademark), which is manufactured through a multistep synthesis, is very expensive and has an issue that fuel crossover is great. Furthermore, a problem of being low in softening point and unable to be used at high temperature, a problem of after-use disposal process, a problem of materials thereof being difficult to recycle and so on have been pointed out. Furthermore, as a polymer electrolyte membrane low in cost and excellent in membrane characteristics which can replace Nafion (registered trademark), hydrocarbon based electrolyte membranes have in recent years been being developed more and more actively.
However, those polymer electrolyte membranes all have a problem of the chemical stability falling short when used in a polymer electrolyte fuel cell. The mechanism of the chemical degradation has not been sufficiently elucidated. However, it is conceivable that hydrogen peroxide generated mainly at the electrode during electricity generation or hydroxy radicals generated by the aforementioned hydrogen peroxide reacting with iron ions or copper ions present in the membrane cuts polymer chains or side chains so that the polymer electrolyte membrane has a reduced membrane thickness or becomes weak. Moreover, there is a problem that, as swell and shrinkage occur repeatedly with changes in humidity, the weakened polymer electrolyte membrane breaks resulting in failure of electricity generation.
Under such circumstances, compounding a perfluoro based electrolyte membrane or a hydrocarbon based electrolyte membrane with an antioxidant to improve the mechanical strength and the chemistry stability and better the durability is being considered.
For example, International Publication WO 2008/102851 proposes a polymer electrolyte membrane in which a perfluorosulfonic acid based polymer has been compounded with a polyphenylene sulfide (hereinafter, sometimes referred to simply as PPS), which is a sulfur-containing polymer, and a polybenzimidazole (hereinafter, sometimes referred to simply as PBI), which is a nitrogen-containing polymer.
Japanese Unexamined Patent Publication (Kokai) No. 2005-350658 proposes a polymer electrolyte membrane in which a perfluorosulfonic acid based polymer or a sulfonic acid group-containing polyether ketone based polymer (hereinafter, sometimes referred to simply as sPEK) is compounded with polyamic acid or polyimide.
Japanese Unexamined Patent Publication (Kokai) No. 203-80701 proposes a polymer electrolyte membrane in which a perfluorosulfonic acid based polymer or sPEK is compounded with insoluble PBI particles.
Japanese Unexamined Patent Publication (Kokai) No. 2004-55257 proposes a polymer electrolyte produced by molding, through heated pressing, a mixed particle obtained by precipitation after synthesis of insoluble PBI in the presence of sulfonated PPS.
International Publication WO 2006/67872 proposes a polymer electrolyte membrane in which a polymer electrolyte and PBI have been mixed and therefore an insoluble PBI particle is contained.
However, as for WO '851, the durability is not sufficient.
As for JP '658, although improvement of durability is intended but not sufficient and the electricity generation performance is also insufficient.
As for JP 701, although durability of the polymer electrolyte membrane is able to be improved to a certain extent, further improvement in the long-term durability is desired.
As for JP '257 and WO '872, durability is not sufficient.
Thus, the conventional polymer electrolyte membranes are insufficient as a means of improving economy, workability, proton conductivity, mechanical strength, chemical stability, and physical durability, and have not been able to be industrially useful polymer electrolyte membranes.
In view of the foregoing background, it could be helpful to provide a polymer electrolyte membrane, a catalyst coated membrane, a membrane electrode assembly, and a polymer electrolyte fuel cell which are excellent in practicality and which have such excellent chemical stability as to be able to withstand a strong oxidizing atmosphere during operation of the fuel cell and are able to achieve excellent proton conductivity under a low-humidified condition and excellent mechanical strength and physical durability.